Spark plugs are attached to, for example, internal combustion engines and used to ignite an air-fuel mixture in combustion chambers. A spark plug generally includes an insulator having an axial hole that extends therethrough in an axial direction, a center electrode disposed in the axial hole of the insulator, and a metal shell that is provided on the outer periphery of the insulator and provided with a ground electrode. The spark plug ignites the air-fuel mixture by causing a spark discharge between the center electrode and the ground electrode.
When the spark plug has a through hole that extends through the insulator in a thickness direction thereof, there is a possibility that the insulation between the center electrode and the metal shell will be insufficient. As a result, when a high voltage is applied to the center electrode, a discharge through the insulator (also referred to as a “through discharge”) may occur between the center electrode and the metal shell. When the through discharge occurs, there is a possibility that a spark discharge will not occur between the center electrode and the ground electrode. Accordingly, Japanese Unexamined Patent Application Publication No. 2012-185963 proposes a method of determining whether or not the insulator has a defect, such as a through hole, by applying a high voltage to the center electrode in a high-pressure atmosphere.
However, the inventors of the present invention have found that there may be a case where no through discharge occurs in a high-pressure atmosphere even when the insulator has a through hole. In this case, it is difficult to reliably determine whether or not the insulator has a defect by using the technology described in Japanese Unexamined Patent Application Publication No. 2012-185963. Accordingly, there has been a demand for a technology for reliably determining whether or not the insulator has a defect.